Analog amplifiers have many applications in communications systems, often being found in transmitters and receivers. At a transmitter, for example, an analog amplifier may be placed between a digital-to-analog converter and a transduction means (e.g., an electro-optic converter, an antenna or a microphone), while at a receiver, an analog amplifier can be found between the appropriate transduction means (e.g., an antenna, an opto-electronic converter or a loudspeaker) and an analog-to-digital converter. An analog amplifier may also be directly connected to a metallic transmitting or receiving facility such as a twisted pair cable or a coaxial cable.
The quality of an amplifier is often determined by its "linearity", i.e., the accuracy with which the amplifier transforms a given input signal into an output signal of proportionally larger magnitude. However, nonlinear behaviour, i.e., the production of distortion components or simply "distortion", may arise due to imperfections in the semiconductor devices forming part of the amplifier or because the properties of these devices change significantly as a function of instantaneous signal level. A more severe form of distortion, known as "clipping", arises when the input-output characteristic of an amplifier is linear only up to a certain input level, above which increases in input signal level will not produce further directly proportional increases in output signal level.
In the case of a transmission system used for sending analog signals such as voice or video, distortion can manifest itself in varying degrees, ranging from subtle reductions in the perceived quality of music signals to the total loss of intelligibility of voice signals, and so forth. In the context of multi-level digital transmission systems (e.g., quadrature amplitude modulation (QAM), vestigial sideband, single sideband or discrete multi-tone systems), distortion can cause degradation of the signal to the point beyond which the digital data becomes corrupted.
In addition, distortion products generated in a transmitter can appear outside of a frequency band assigned for transmission, thus causing interference to other signals sharing the same transmission medium. At a receiver, interfering signals outside the desired band are usually removed by filtering. While a significant improvement in performance can be achieved by using filters, such filters can become prohibitively expensive when the transmitter amplifiers induce large amounts of distortion.
Moreover, in a modern telecommunications system, it is not uncommon for amplifiers to be fed with signals having frequency content in the Megahertz range. Thus, analog amplifiers are frequently expected to perform well at high frequencies. However, it is known that amplifier open-loop gain drops off with frequency, and therefore little gain is available as the frequency of operation increases. Since the amount of distortion that can be attenuated is a direct function of the amount of open-loop gain available (when the amplifier is used in a feedback configuration), a reduction in open-loop gain reduces the capability of the amplifier to attenuate distortion, and hence it follows that the deleterious effects of distortion are magnified at higher frequencies.
In view of the above, there is clearly a need to limit amplifier-induced distortion, particularly at high frequencies. Unfortunately, various prior art attempts at doing so are unsatisfactory from the point of view of performance or cost (or both). For example, it would appear obvious that using components with improved linearity would solve part of the distortion problem. However, higher quality devices are not only more expensive, but unless their output range is increased, such amplifiers will still clip, causing the aforementioned distortive effects. On the other hand, increasing the output range of an amplifier while preserving its linearity entails a huge increase in cost, defeating the practicality of such an approach.
Another seemingly plausible solution is to add subsequent amplifier stages and to apply negative feedback around one or more stages. However, this requires that the open-loop gain be much larger than the closed loop gain of the amplifier stages equipped with feedback, a condition which is difficult to meet at high operating frequencies. Clearly, the onset of distortion remains both a common and serious problem, for which no adequate solution is taught by the prior art.